Hi,
Welcome to the forum, sadly it's not as easy as just putting guinea pigs together. Firstly, a new guinea pig should be quarantined to ensure it shows no signs of illness and actually in the event of your existing guinea pig it's probably a good idea to keep them separate for two weeks - again in case of signs of illness. for the benefit of doubt - this can be hunched up in the corner, not eating and doing non solid healthy looking poo's.
Anyway, since you have your new pig, you need to bath them thorough to remove any previous smells, you can use a none chemical shampoo - i.e. natural, or you can pop along to gorgeous guineas web site and order some of their fabulous shampoo - oh yes we even have shampoo!
After bathing...do go along and have a look at the care section on How to do these things.
The enclosure should have been washed and disinfected thoroughly including bowls, bottles, bedding etc to ensure no smells are left over - milton sterilising fluid is good for this ensuring it has been thoroughly rinsed.
bonding needs to take place in a neutral zone - no smells without any houses in i.e. no one entrance. There is a thread on here on how to bond actually I will try and find the link for you and suggest you have a good read.
Failing to bond properly will mean that they will always fight.
Oh yes and make sure they are dried off - your pigs so they don't get cold and give themselves a chill also.
This is quite a short notice but didn't want you thinking no one was around to answer your questions, anything else just shout up.
You can only put them in the hutch after they are bonding properly and aren't fighting at all. It can take anything from 1hr to well... all day it really depends on the personality of the pigs.
good luck!