Situation-dependent.
I had one scenario where I was removing a sketchy tree, so I climbed up one, limb walked out to take the top, came down, climbed up a pine on the opposite side of the tree, set a TIP, then using both climbing systems in each tree on either side of the dead one, lowered myself into the middle and lanyard in to the dead tree while I pieced it out.
It was a lot of work, but I also happened to be doing some pruning on the other trees and had to climb them anyways.
Spiking up the dead tree while tied into another certainly is an option. If the rot and decay is so bad that your spikes are just going right through or getting stuck, perhaps you climb more manually if there’s enough branches to allow. Or you could use a secondary climb line to help advance one’s way up the unhealthy tree while having that backup line in another.
Ultimately though, as the climber you have to feel confident in what you’re doing. So even if something may work for someone else, doesn’t necessarily mean it’s right for everyone. Just a thought.