Take a look through some of the mercatoris journals (Forums->Journals and Photos->List of Our Octopuses 2008, 2009 and 2010 provide links back to the journals). GHolland has the best collection of photos starting with
Varys and forward linked to her children and grandchildren. There are additional photos in
Trapper and her offspring posts that may help ID them as mercs. Briareus will grow arms close to 10x the mantle length, merc's arms are quite short and only about 1.5x mantle length. Mercs arms reach proportions early but very young briareus octos are much harder to identify using arm length as a criteria as it takes awhile for the extended length to be noted (the webbing is present fairly early, as are the green fluorescent dots). If you are seeing arms thrown over the head with one or two placed either between the eyes or encircling one eye as it peaks out, these are two very characteristic poses for the mercs. If you shine a light on a briareus, you should see green fluorescing spots all over the body and arms.
If you determine these are mercs, then all the suggestions to separate them can be thrown out the window
. Both could be housed in one 20 gallon cube and keeping them together may produce a more active pair of animals. If you decide to introduce them, you will need to watch them carefully much of the night to be sure they don't fight seriously. There are two differences in your pair than the ones we have successfully kept it that they are not equally sized and have not been living together. I would completely disrupt the tank if you combine them so that neither has an established home. If you can come up with some Giant Purple Barnacle shells, these seem to be a preferred home for mercs (even though this barnacle is not native to FL). My females have all taken these as a residence inspite of the fact that I can see them at any time by peaking in the barnacle shell.
Unfortunately, unlike the briareus, mercs don't come out during the daylight (at least not for long) until they are about a week from the end of their lives. The do very well under red light (even fairly bright red light if it is left on 24/7).