The red growth is the named form and the green is the species understock. Which you decide to remove is up to you, depending on what the remainder may look like after removal. Most folks want to save the named form and remove the rootstock plant but ideally that happens as soon as the rootstock growth is noted. If it is left too long, it can get too large to remove easily or have aready affected the growth of the named tree, so then the opposite approach may be considered.

the first is undulata albomarginata .. albo=white marginata=margin or edge the second remindw me of lancifolia ... two very old workhorses in the hosta world ... ken

I agree that it’s best to cut out the damaged stems since the rest of the plant looks fine. Sterilize the clippers between cuts with a bleach solution or wiping with alcohol. It looks like either disease or damage from something like severe winds or a chewing critter may have caused damage, so I would likely spend some time looking at the lower stems to see if there is evidence of borers or fungus or gnaw marks. I might also push down gently on the surrounding soil to see if there is evidence of burrowing critters that might have damaged roots. I have never seen this type of problem in panicled hydrangeas. How long have yours been in your garden?

we have all seen two plants in one hole... from bird dropped fruit.. to 2 acorns ... to two seeds with annuals ... [or a thousand maples in a gutter.. lol ...] imo ... that is not an issue .. as long as both plants are basically equivalent with future growth potential ... obviously.. if one will dwarf and outgrow another.. forcing it into shade ... then that one might not thrive.. nor be much of a pollinator for the huge one.. lol ken