Second guinea

We have had a guinea pig for a year now, and plan on getting a second at the end of the month.

Is there anything we should know or take into account when getting the second piggie or introducing them to our current pet?

We have a female who's 15 months old, and plan on getting another female. We just bought an extra-large cage (40X26) and still have our old cage sitting around, it's half that size.

Some factors we're already considering include: separating the two of them until they've both been checked by a vet; slowly introducing them but only while we're around to supervise; how to lay out the bigger cage (two food and water dishes?) once they're together.

Any feedback would be great! Thanks.

This thread is a month old but maybe you haven't gotten another pig yet. So here goes...
Before you get another piggy I would confirm the sex, and health of the piggy you already have from a good gp vet.
Out of the 5 piggies I have, only 2 of those were sexed correctly when I got them. Two of them were supposed to be female and were indeed male. The other was to be male and was female. So now I have one more piggy to get so that all of mine are paid up (as soon as the final male has been neutered)