Trouble introducing two male guinea pigs

I bought a male guinea pig about six months ago. At the time, we couldn't afford to get him a cage mate. Now that we have the resources, we bought a bigger cage and an immature male guinea pig. We gave the new guinea pig a day on his own to get used to his surroundings, and then put our older guinea pig into this new cage with the smaller one. At first, the older male was mounting the younger one continually, as well as rumblestrutting. Today is the third day we have had them together, and the mounting has subsided a little, but the rumblestrutting is almost non-stop. They have not bitten or fought with one another, though. The younger one basically tries to run away from the older one when he does this, but every once in a while, he will attempt to mount the older one. I'm worried that this isn't allowing either one to assert their dominance, which is why the rumblestrutting and bullying is continuing. I wasn't sure whether I should be worried (or if this is normal), and was also wondering how long this should go on for. We have two weeks to return the new guinea pig, which we'd really rather not, but if they're not going to get along, then we figure that is our best option. Please let me know your advice! Thank you.
   

It sounds like they are still trying to sort out dominance.
   
If you are coming to the end of the time when you can return the second one and things haven't shown a marked improvement, I'd be tempted to return the new one, but I don't want to tell you what to do.
   
That is your decision and they could very well work it out and be pals for their entire lives.
   
Two boars can live together quite happily but if they are not getting along, neutering will not help that.
   
And do not get a sow. Boars who get along will fight over a female.