Summing up L & R line-level for dual subs

I have dual subs and I'm taking the line-level signal out of my Lamm LL2 (pre) second set of L & R outputs. I'm setting up the subs following Earl Geddes approach to multiple subs setup, that calls for the summed-up mono signal played by all subs, and each sub set up to play a different range.

I've seen Y-type connectors suggested as a way to sum-up L & R. Would that work?

Thank you!