Creating pools with mixed services in GTM?

if one shouldn't be available without the other you can either add the opposite monitor (sip to the https vip / https to the sip vip) to the pool at the LTM level and use the bigip monitor on the GTM, or if you serve the content separately for non-GTM use, you could add the https virtual server to the GTM pool and then apply a sip monitor to that pool so that GTM gets the https status from the LTM via the bigip monitor and checks the sip status itself.