speaking from personal experience, I haven't had any problems with doing a topper immediately after #20. I once did (before the Zaino days with my daily driver) a coat of #20 followed by a coat of Gold Class Liquid, both applied with PC. The wax lasted about a month, but the #20 kept me through the winter (3-4 months)....
That was about the time I switched to Zaino, although I'm sure my #20 would've still lasted me.