Trying to understand - they are not actually trips for work, but are social/networking ones, like a 5 day trip to the Hong Kong Sevens or a corporate entertainment invite to the Miami GrandPrix? If the latter, then I think you can reasonably sit with him and state that you feel more than two jollies a year (and, even if he is ‘networking’, they are jollies), unless he agrees to take you on some of them too - as usually for overseas trips spouses are often included in the invites and it feels as though he is excluding you. Additionally I think they are a taxable benefit if client funded and usually most firms require you to get HR permission to ensure that all benefits and freebies from clients are logged. MY DH has to do this as they ensure that the ‘treats’ are spread across all members of a function and not taken up by one individual manager.
If they are business conferences, however, and he is there at the expense of his company and representing them, then I think you need to ask whether he can decline to go and recommend another team member go occasionally. My DH does this as, frankly, they become very samey and are rather draining. He should have to agree with his management how many he should attend - and again, there are usually guidelines by HR as these too (if they involve travel outside work hours, five start hotels etc) begin to fall foul of taxable benefits so I would be surprised if he was going on lots and lots in the company’s dime - depending on his industry?
In all circumstance, I think you need to sit down with him and clarify what type of trip they are, who is paying, who is attending, whether spouses can be/are included in the invitations, how does he expense them and whether HR/Management authorise them. That level of transparency should be the norm in a functioning marriage.