Just curious but are you wanting to use two threads to make the quilting more "in your face" or are you doing it to combine colors? If "in your face" why not try Wonderfil's spaghetti thread? It is a 12 wt you can run through the top on your longarm. Well supposedly. That is what the wonderfil people told me and they had samples there that Kim Stotseneurg had done (she quilts on an A1). It definitely gives dimension and pow factor to quilting and it stood out fabulously and they have some great colors to choose from. I have yet to try it myself but I bought several spools with the intent to try. I am thinking this may be a good alternative to double thread for you and your robotics. Especially if your design has backtracking involved.
Another option to consider with heavy decorative threads, is load it in your bobbin and load your sandwich upside down. I have another friend who does amazing bobbin work on her Innova and uses all the pretty fancy threads like Razzle Dazzle and Candlelight.