I use what Home Depot calles 'paver underlay' its has some gravel and sand in it ...then I add some sand to the underlay and some peat. They are not all that picky as they don't burrow much if at all. They do like to hide so provide things for them to hide under.
If you only have one you can put it in a smallest pet pal or the next size up from that.
I usually mix some of it in in most of my desert enclosures and sometimes into other substrates when I feel some rock and other fine material might help. Its not great for burrowing but it is much better than straight sand.
Funny you should mention M.martensii. I was at my girlfriend's parents house for a week .. just got back. Anyhow, the local pet shop up there had some scorpions. Nothing too fancy, but better than most. Anyhow, I was looking through and ended up taking a M.martensii with some missing distal tarsal segments on two legs. The guy was honest and up-front, telling me of the damage before I even looked. Pictures will follow when I take some.
On another note, the shop was good in providing scientific names and I noticed that one was labeled "Grophus madagascarensis." I took a look and found an Ishnuridae staring back at me. I told them the ID was wrong and they didn't fight me or anything. I phoned them back later when I was pretty sure it was an Opisthacanthus madagascarensis. That would certainly explain the source of the error. I also gave them the URL for the scorpion files and they seemed sincere in their gratitude for the info.
The short answer is yes. The long answer is that, as with any scorpion, there is always the risk of canabalism. I understand that this species is pretty good at not eating each other, though, and the Rubio book has them listed as communal.
Two in a five gallon tank is not a problem. I had 4 in that same size enclosure and probably could have kept six or even eight had I provided enough shelter...though they seem to like to clump together. If any of your M.martensii have babies..( 2of my 4 did)...you need to remove the mother with the babies immediately as the other adults will try to eat them. It would probably be late March or early April if that were to happen. That is assuming that you have a gravid female and the conditions are correct. Of course telling if one is gravid is another thing...I had no idea they were gravid...didn't look it as far as I could tell. Anyway...they are cool semi-hot scorps that are very active at night.