From what I've read around, it's hard but not impossible. If you have 2 males from the same litter and keep them in an estimulating environment (big cage, change things around often), then it's possible, but still highly probable that will end up fighting. I personally wouldn't risk it. Still, with a lone male mouse, you have to keep a stimulating environment cause if not, the can get depressed, cause mice are social.
Gerbils, even though they are hard to introduce, are highly social and prefer company. 2 male gerbils can be kept together no problem and in fact, females are more territorial and have a higher probability to fight, even if they are already bonded. A lone gerbil tends to get depressed, not be as active as normal, and usually get fat. Have you considered them? In a 20g you can have 3-4 males, better if you pick them very young and/or from the same litter and you can choose from different colors. 8)