How long should I wait before introducing my two gerbils?

[Maggiiee:)]

I have one gerbil named Spice, who I've had for about 10 months (but I'm not sure how old he really is). He's small compared to the new one I just bought cause his tail's width is definitely smaller than my new one. I got another gerbil yesterday and his name is Pepper. Pepper seems more like an adult than Spice cause I noticed that Pepper's size is dramatically different. His poop is bigger and hes fatter (not that I don't feed Spice, cause I do...) So doing some research, some people said to do the split-tank method but I dont have a tank. What I do is put them in seperate cages and put the cages near each other so they could see each other and sniff. They've been sniffing a lot and seem interested at times. I switch them into each other's cages occasionally and it seems like they're interested in each other. When should I actually put them together? If pepper and spice don't get along, I have to return pepper so how long do you suggest that I wait? I can only return within 15 days... I'd hate to return Pepper but if it doesn't work out, I'm going to have to. Any suggestions?

p.s: When pepper is in spice's cage, he seems overwhelmed but when spice is in pepper's cage, hes very curious and less overwhelmed.

Spice is like the active and energetic one while Pepper is the shy and scared one.