Using bi-amp in a 2.5way build to avoid a low impedance?

So, I am building a fun 2.5way tower speaker with 2 different bass drivers: the Wavecor WF152BD 4ohm 6" midbass, and a SBAcoustics SB17NBAC-8 or -4 for the lower woofer. The Wavecor will be the main driver in a sealed compartment, and the SB will be either in a separate sealed or maybe ported enclosure (TBD). The SB is really just there to fill in the bottom punch and offer the baffle step correction. Both woofers will be wired in-phase.

If I use a typical parallel crossover, I am looking at a pretty low impedance with two 4ohm drivers in parallel, so I would avoid that. I guess I will try the 8ohm woofer, but that still leaves me with a 3ohm dip right at 150hz. Below is the high-pass crossovers just for the two woofers. (Wavecor is in Green in the Tweeter section, SBA is in Blue in the Woofer section, sum in red. Tweeter is not profiled here, and yes I need to move to a better modeling tool, but I can use this one on my mobile while traveling for work):

I am just using standard FRD/ZMA files at first, to figure out the basics of my design and what components to order for the first try. Actual measurements will come later...

I was also considering creating a bi-amp setup, with one amp running the tweeter and Wavecor, and the second amp powering the SB driver. I think this makes it a bit safer? As each amp channel only sees one woofer.

My Marantz receiver can re-assign the surround amp outputs to be additional mains outs, giving 4 discrete mains channels:

I was going to try that setup. But I have no idea what this statement in that link means: "This connection enables back EMF (power returned without being output) from the woofer to flow into the tweeter without affecting the sound quality, producing a higher sound quality."

I know what Back EMF is....I just don't understand it in this context... It seems backwards. I would think it should isolate BackEMF produced from the woofer motion in that channel, and not allow it to Flow Into the other channel.
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I wouldn't put much stock in this, just bi-amp for the reasons you intend. Bi-amping was kind of a solution looking for a problem and so was often overstated, even when it wasn't necessary.

By the way, the impedance you show isn't that bad. Some amplifiers will be fine with that.
I've bi-amped most of my 2.5 way speakers with separate power amps as well as using the bi-amp option in a HT receiver. I prefer an easier impedance there are amps that do not like low impedance. Amps like the A-S1000 and 840A tended to go into protection mode with a 3.6 min impedance where an old AKSA 55 had no issues. The other consideration is both woofers are different (like mine) so would behave differently unlike a standard 2.5 way using the same woofer.

That EMF statement doesn't apply to your situation as you are bi-amping a TM + W and not a T + M.