I'm not a fan of hard attachments, but if you each held the end of a buddy line in your hand, that would work.
Working on buddy skills, which were probably overlooked in your OW, would solve the problem. At first keeping track of each other will take a lot more of your attention than you would like, but over time it will become second nature, Swim side by side so a glance will spot your buddy. Both of you have to be committed to working togather. Before you take off to look at anything, signal your buddy so he comes along. Remember that the dive belongs to both of you, and your interests should not overshadow his. If you do need to go single file, be careful, this is where a separation is most likely to occur.
Keeping track of your buddy is one side of the equation, the other is insuring that you do not move fast or irratically. so you are hard to keep track of. Practicing these skills may screw up a few dives while you sort it out, but will be worth it in the long run.
In clear water there is more room to separate without losing your buddy as viz decreases the buddy pair has to be closer togather, to the point of using a buddy line. Most times I have used a buddy line, it was more a working dive that had to be done rather than a pleasure dive. I can keep track is someone in 5' viz, but by then the diving was not fun,