Fairly simple, use your Force of Nature spell in a spec you have as one of your two, and you will keep them even if your second spec doesn't have the talent in it. The primary tree (the one you actually control, with the other two following its actions) will despawn, however. They will be stuck in whatever stance you had them in previously, aggressive/passive/defensive.
Example, balance as your primary spec, resto as your secondary; cast Force of Nature, swap to resto. Be in tree form with 2 trees next to you!
(Not at a computer with WoW at the moment, will post a SS when I can)