Limit the time of the first initial get togethers. Have a definite start and end time for the 3 of you to be together. Talk to both beforehand about how long they would like these initial encounters to be.
Also set up a schedule of date times for each of them and then stick to it. You and your husband are going to need to recconect after he's been away and your boyfriend is going to need reassurance that he is still important to you. Discuss often with both of them how the arrangements are working out for them and then tweak the arrangements as required.
Everything will be ok in the end. If it's not ok it's not the end.