Hi Patti - Sorry I just ran in to this post. As you are likely aware, Serviceberry can have multiple trunks popping up as sports off or near a main trunk. They also have a rather frenetic growth habit. Helter-skelter.
Short answer to your question is Yes. But I would not do it all at once. Instead, try to spread it out over a period of time. A bit now, a bit more next Spring, some more Fall of '16 etc. By 18 months or so, you should have it shaped the way you want. Then do regular pruning to maintains shape. On a regular basis (annual is recommended for Serviceberry), pruning should be done in late winter/early spring.
While Serviceberry is a rather strong tree/shrub, like most things it has its limits WRT heavy pruning.
I have to say, that is a rather odd growth form there. Heh
I would start by cutting back the tops of those two Ys in the pic above. You will likely trash a lot of blooms as it blooms very early on old wood.
