I'm in a similar situation (5lbs to goal). I was at 10mg Tirz, did my first pin of Reta (2mg) last week and dosed Tirz down to 7.5 today. My plan is to titrate Tirz down to zero over the next 3-4 weeks and titrate Reta up to 6mg over the next 8-12 weeks and try it solo for maintenance.
If my heart rate doesn't cooperate I will stop the Reta and dose Tirz back up, hopefully to just 7.5mg and stay there. I will adjust dosing schedule based on feelz/weight.
This is my plan, but you know what they say about plans.