DBT doesn’t seem to be basking anymore

For a number of weeks now, I haven’t caught my DBT up on the basking platform. My RES is up there a lot. Is she hogging it? Is there a territorial thing going on I don’t know about. I moved my RES to a Rubbermaid tub for a few hours or so, but the DBT still didn’t bask. Do they bask less in the wintertime, even though they’re inside? He still seems healthy. He used to bask ALL the time. p.s. the RES is a lot larger than the DBT and there is only room for one of them at a time, now. Do I need a larger basking area/tank? Walt Chandler, AZ