So far as I know, you can't, even if you could there are two things to point out, firstly, Mandau is the superior off-hand for Excalibur anyways, its the best one there is overall so just do that if you want Dual Relics. Secondly even if you could do Dual Excaliburs, you would not be able to get both to 119 most likely due to the fact they are R/EX, so the level gap is probably the only reason it would probably work.