As long as the B18 guides are in good condition, the oil seals for the B18 will work fine. They sit on top of the spring retainer. If you are changing to double springs, that retainer will not be used, and more than likely the B18 seals will be useless. That means new B18 guides will have to be fitted, with the ends machined to accept the B20 seals - a simple job. But all this adds up.
My advice would be to have the seats installed, and then install stock new springs, and use the B18 seals.
Double springs increase the load on the cam to lifter, and that's the last thing you want, and they are normally used to provide different harmonics when the engine is to revved up a great deal. For most folks the stock springs work fine.