what code are you using? if you're using B31.1 for example, you can only have one SIF. so yeah, I'd choose the highest one.

If you model the dummy pipe directly to the bend node, then CAESAR will already know which way is inplane, which way is outplane since there are 3 elements attached to the node.

If your modeling methodology involves adding one rigid element connecting the bend node to the dummy pipe, then CAESAR doesn't know which way is in plane and which way is outplane. You may either use the highest SIF to be conservative, or you may have to model another rigid element to "trick" CAESAR into correctly determining the inplane and outplane directions. To make your life easier if you use this dummy modeling technique, just use the highest SIF unless you find it to be too conservative.