Port trunking & stacking

The two stacked swicthes may not even allow the configuration you propose - many switch stacks require all 802.3ad trunk groups to have all the links on the same swicth. I think this is beacuse even though the stack gets managed as a single device, as far as moiving packets through the swicth is concerned, the stacking port is just like any other port, and the two switsches in the stack are just two connected sitches. Many go even further that than and require them to be in the same group of 8 or 16 ports on the same switch. I thhink that is because they use 8 or 16 port switch ASICS and just stack them internally.

I am unfamiliar with the Dells, so check your manuals. Hopefully none of the above applies to your situation.

Otherwise, to answer your qusetions: no, you should not need spanning tree, and yes, assuming the trunking is 802.3ad, it will continue to work if one link goes down and there is only one link left in the aggregate.

If it's nopt 802.3ad but some proprieary trunking, then I dunno.