None of the apricot rootstocks dwarf enough to make much difference. Growing conditions, some of which you can control, and summer pruning are your major ways to control tree size. I don't know that they will bloom together. It depends somewhat on your weather. I've seen both listed as being late blooming for a cot. I haven't grown Mormon. But can say that Tomcot does very well for me and many others who've posted here.
The Fruitnut
Q