The effects don't stack either way, as long as you have the bubble up long enough to survive the four ToM bursts (the length of the buff is unlikely to matter, the ToM will burn through the available damage buffer before time is a problem) and maybe allow time after that so that people can gather in the middle before the runner arrives with the aura, you should be fine with just one bubble at that phase.
Typically we have two titans in the middle (just in case) but only one of the titans uses their bubble, which keeps the second to use in the shade if necessary. Or you could go for both weapons/blessings in the middle, just beware that you'll eat through the blessings buff quicker.