There is the "Allow Assumptions" for the case of fasteners that have no rotational alignment requirements. Often people will even leave screws rotationally free. No big deal. But if you have something that does matter rotationally like this leg, you need to click off the Allow Assumptions button and mate the orientation. The fact that this is on by default is what bothers me. I'd rather choose it when appropriate.
If you remove the Allow Assumptions, you should be able to add a 3rd constraint. You may need to click on New Constraint or it will overwrite the previous one. This assembly dialog lacks a lot to be desired. I find it cumbersome as all get out.
In general, yes, you can affect each contraint by their own merrits. The dialog use to be even worse, if that's possible 
There was no allow assumptions to check or uncheck. It figured it knew what to do with only the two mates.
I had thought about that, but when I checked it wasn't there to uncheck. 
sorry, I worded it wrong earlier: It IS fully constrained.......it's just that you don't like the default "solution". 
I found out recently that if you use two inserts constraints, you need to check the second constraint because it will default to oriented (Creo/WF5) instead of coincident because it is assuming that you alligning a bolt pattern and the first insert can be coincident but second in that usage will me oriented so you are not over constraining a part. Since my inserts are perpendicular, you can have two coincident mates.
I have started using two planer orient mates to completely constrain the part leaving no run for assumptions.
My 2 cents:
I'm not sure of how it works in Creo1.0 or 2.0, but in WF5 there are actually 4 ways to assemble 2 parts with insert+insert constraints (I've added align/mate constraints to get rid of default Pro/E placement):
