ERMEGM,

nibble1 var byte
nibble2 var byte
both var byte

nibble1 = 7
nibble2 = 10

'to store the 2 nibbles:
both = ((nibble1 & $0F) << 4)
both = both | (nibble2 & $0F)
'both now = $7A

'to retrieve the nibbles
nibble1 = both >> 4
nibble2 = (both & $0F)