[sdiy] How to continually blend from a parallel connection of the two multi-mode resonating filters?

[Dave Leith]

I own a Sherman Filterbank (http://www.sherman.be/) which has a nice
feature where one can continually blend from a series to parallel
connection of the two multi-mode resonating filters. Haven't seen this
elsewhere and it's a nice feature and never seen circuitry for this.
Other features which are quite standard is blending from HP->BP->LP -
it's not this feature I'm interested in...

Any ideas how this could be done?

Thanks
Dave