Then you need to remember a set of selections under a name that will be common to both markets. In this case, both markets have the same starting time, so remember the selections you need under a name that contains the event_start variable, e.g.:
mysels_event_start
Then you can address the set of remembered selections from both markets, e.g.:
mysels_event_start_r_1_back_price
But make sure you do not add two markets (two different horse races) that start at exactly the same time (a rare occasion, but it happens).