So I decided to convert one of them to a two strap suit! Here's how it started.
I checked the suit straps....getting a good idea of how to far in fro the sides I needed to go and how tight the straps needed to be. I pinned them with safety pins so I could try this on without worrying about stabbing myself with straight pins or pulling them loose.
I hand stitched these down - using a backstitch across the width of the strap. Then I used a whip stitch to secure the strap down against the top of the suit. I wasn't sure about whether the placement was perfect, so I wasn't ready to cut the last bits off the straps till I wear this a few times to be sure. I think you could machine stitch and be perfectly fine. Make sure you secure the ends and stitch this down tight - nobody wants an unfortunate swimsuit accident :)
I can't wait to convert the other two suits the same way now that I know this works!