It is possible to do a sand casting with a one piece pattern, lay the pattern in teh bottom of the drag box, pack sand around it in the usual way but when you turn it up the other way you need to carfully carve away the sand down to the half way point. You then dust with talc and ram up the cope as you would with a 2 piece pattern. First casting I ever did was done that way using a Mamod flywheel as the pattern. Gives a perfect finish to both sides.
Having the two halves on match boards is the ideal way for production or small batch work buy one offs or short runs it is as quick to do them without a board which is what I have done for the last few I have had cast. Also means the foundry can often squeeze them into the corner of a larger box to keep costs down. Just use pegs and sockets to line up the two halves. You can just about see the pegs/sockets on these.

Edited By JasonB on 04/03/2018 13:14:08
Edited By JasonB on 04/03/2018 13:21:05