Hey, you're going to need an additional heat source. Not another UTH. Heat mats are designed solely to provide a hot spot to aid in digestion. They're not made to raise ambient temps. So you'll need a heat lamp with either a bulb or CHE (ceramic heat emitter). Or you'll need a RHP (radiant heat panel).
Bulbs are the cheapest, but last the least amount of time. CHEs are a little more money but last MUCH longer and don't shine any actual light. RHPs are definitely the most expensive, but definitely the best option.
Another heat mat simply won't do the trick.