Yes, you can do this. Grohe and Hansgrohe are two well known brands that can be used to acomplish this (I know, since we're using them both). as qdognj explains, you'll need a couple of things. 1st, a temperature control valve. Thermostatic is what's recommended today, and controls hot/cold. The second is you'll need volume control or controls. With grohe usually you get one volume control for each showerhead/outlet. It's a single knob that opens the flow to a given head. Hansgrohe makes a single control that does both temperature and volume to different outlets. It's less hardware (and expense) but less flexible. If you're using a standard shower/bath setup, you could use the hansgrohe pretty easily. But if you have a fancier shower stall, grohe gives you more flexibility, but at greater expense. Both companies make these basic products in a bunch of trim styles (modern and retro) so you can choose what you want.

Before you try to replace anything call Grohe customer service at 1-800-444-7643 and discuss the problem with them. They have amazing customer service and not only will probably replace the cartridge free of charge but can give you and opinion as to whether its a DIY project for you. Alot of the process has to do with access to shut off valves, the proper tools and having the right parts.

I purchased Grohe: 35-026 GroheFlex rough in valve, 28-448 Shower Head, and a 19-849 Thermostatic valve with diviter. Plus the tub spout - and was somewhere around $1000 for a shower/tub setup. I figure a licensed tradesman - will want atleast $60/hr - I'd say more like $75/hr - and I'd guess $125 for 'spot' work. The guy probably has 50% overhead for his truck / business insurance / health insurance / tools / overhead. A bathroom is probably the most expensive room in the house. You want the valves installed correctly in the tile walls, level, to the proper depth, etc. Have you seen his other work? Or spoken to previous clients? Is he going to do the work (I hope not sub it out, or give it to a 'helper'). I don't know how much material he is going to supply (fittings / PEX / copper / PVC) , etc I think you would feel better in getting another estimate - but it may be apples to oranges - since you are changing the original design.

Let me add to an old thread. I started using Grohe in my projects back in the late 90's. Back then some USA companies had liftime coverage against leaks ....Moen was one of the few that would provide finish for chrome if pitted. Not all the companies provided lifetime .. Kohler for example was a pain. Anyway, by 2000 Grohe provided lifetime if you had the paperwork proving it was bought after (1997). Since I did a lot of projects for family as well -- we started seeing some failures in the later 00's and Grohe was always great with sending parts. Mostly it was the main cartridge failing in the shower or kitchen faucet ... we also had other issues with the hoses and heads of the Ladulux faucet. The head is plastic and the switchover from stream to spray would fail -- pull out hose is plastic inside. They were always great with anythng you needed. This also included a few chrome parts for the shower trims. I always recomended them to everbody. About 4+ years ago I tried to get one of the main pressure hoses for a ladylux kitchen faucet that was leaking under the sink. Told it was discontinued and no longer available. That it ... goodby. I had one of these fail on another faucet many years ago and they sent me two so I could replace both. The problem is this is not a standard USA hose -- the faucet side requires an odd metric male fitting that no one carries. The only solution is to trash a perfect 12 year old (at that time) faucet ... top of the line and $600!. Customer service was not nice about it either .... had about the same results when I called regarding another cartridge that was leaking a bit later ..... they gave me the "limited warranty" line and said that after 10 years "or so" it was normal wear. When you buy something with a lifetime warranty ... you expect it to be that. Just to see how they would respond to another isuue ... sent them a picture of a bar faucet from the early 00's that all of a sudden had pitting on the spout and was dripping ... nope -- too old. Thankfully, I was able to get a NOS faucet spout on e-bay for $50 and get this .... someone on e-bay had a special run of the Grohe pressure supplies made with the metric fitting .... that was $90 for two. But, it's anoying that Grohe is not stocking parts. I recenlty bought two condos at the beach in SC and everythng is Kohler from about 20 years ago ... one of the laundy fauctes had a cracked head, the other leaking. They are white and I had trouble matching them up .... I called Kohler for some help and they asked for some pictures. Not only did they send me two new whited heads -- they sent me all the parts to get everythng else in order .... no questions -- no charge. It's a shame ... Grohe made a great product and I installed 100's of them .... but -- I can't recomend them as when they fail in 12/13 years they may not help.