I think you can, and here’s a situation:
You WANT the cruiser dead, but you NEED (as in if you don’t get it you’ll lose the game) 3 of your 5 subs to do whatever. Perhaps mount an attack on some transports defended by 2 destroyers. So you defend with 2 subs, knowing that come whatever dice you have 3 subs left to do sub stuff. You still have the 2 to defend.
Unlikely?
Totally.
Possible?
Possibly.