would the best way (in the sense of being faithful to the Rasch theory) then be to put all the responses in to one joint response matrix as if test A and B were one joint test? (The method you call "Concurrent or One-step Equating" in your Program Manual 3.92.0 (www.winsteps.com/manuals.htm) ).
Can you say that the other equating (joining) methods, which you suggest in the chapter "19.35 Equating and linking tests" in your Program Manual 3.92.0, are not exactly true to the Rasch theory as you have not analyzed all the data from test A and B to fit with One Rasch model?
I do wonder what would happen if there is a Differential Item Functioning (DIF) among the 5 common items with respect to whether the responses come from test A or test B, ie. does such a DIF introduce bias of the person parameter between test A and B?
Celery, concurrent equating is usually easier overall. But always analyze test A and test B separately first. Verify that test A and test B are functioning correctly. Then scatterplot the item difficulties for the common items from the separate analysis of Test A against the item difficulties from a separate analysis of Test B. Verify that they are on an approximately straight line approximately parallel to the identity line. Then do the concurrent analysis. You can also do a DIF analysis on the common items.