Set out rules beforehand, between you and your partner and then between the two partners. You can start by having a look but no touch, or a look and light touch rule. Whereby the two couples are having fun with their own partners in close proximity to each other.

If you feel comfortable you can take it further, always making sure that you have the caveat that anybody can stop at anytime and that you shall remain lighthearted in that situation.

There are many levels to opening your marriage, don't forget that the brain is the largest sexual organ, and sometimes all you need is a hint of a situation to spice up your sex life (not that you suggest it needs spicing up) for months to come! :-D

EDIT: To add to that, if you do find yourself in that situation, take the female of the partnership aside and set the rules out with her first. Then later the two of you together set them out in front of the men; trust me, they'll go with whatever you guys have agreed :-D

Cg

Cg