I think before you put either sow to the boar, you make sure you've got a market for the pigs or space, time and cashflow to allow you to run them on for pork, as Hughesy says. It's very easy to think you'll find buyers but unfortunately it's not always the case - especially if you're competing with established/recognised breeders in your area.
I'd not necessarily rule out having them both farrow at the same time - it can be handy if one sow has a lot of piglets to be able to foster them onto the other (you wouldn't be able to do this if they were 3 weeks apart)
If both sows currently run together there's little chance of them fighting and as long as they're just about to come into season when you put the boar in they shouldn't give him too hard a time either
Good places for information on breeding are the GOS breeders club website and I also like the Haynes pig manual - it's very straightforward and will give you a good starting point. But if you're planning piglets for next spring you'd better get reading all you can or look for a local, friendly breeder who might let you help out with their pigs to get a bit of practical experience - you'll need to be up on your injections etc or else you'll end up paying a lot of expensive vet call-outs.
You'll need to figure out what you're going to do with your boar when he's not serving your sows - they don't do when not 'working'. It can lead to them getting a bit frustrated and it can also sometimes affect fertility levels, so you might want to think about getting an approved isolation area sorted for him and offering his services as a stud for visiting sows (or hire him out for stud) You'll also need to have any young female weaners fairly far away from him, otherwise when they start to come into season he'll get to them (possibly taking gates and fences with him !)
There's way too much involved in farrowing and breeding to write it up as a short 'how to' unfortunately, but read through the pig section and you'll get an idea of some of the most common things
And I'd suggest (if you've not already) get yourself good weather-proof gear (cos you'll be out there twice a day, every day all through the winter) and get your water pipes to the area where the pigs are insulated/buried - freeze up's in mid winter when you've got to barrow water to 20 plus pigs isn't fun
Sorry, this all sounds a bit doom and gloom
Breeding pigs is great and you're helping to keep some of these rare breeds going, but it's definitely a labour of love and there's LOTS to learn (even if you've been doing it for years, you always learn something new) so good luck with it and keep us posted.
Welcome to the world of breeding pigs