It's probably not "siphonage" - likely the bathwater just isn't
draining fast enough to prevent all the pipes filling (to the brim of
the shower waste) with water. I had exactly the same problem caused by
bath and shower being connected to the same tee. Fitting a Hepworth
"Hepvo" trap to the shower branch solved the problem perfectly.

www.bes.ltd.uk sell them; part no. 15122, but you should be able to get
one from any plumbing supplier.

Vaci