booking a flight for a band with guitars as seats

So I know the deal with the 'extra item seat' it's brilliant and should save me a lot of money when I book a band to come over.

But it's not currently because it's not letting me put on more than one extra item seat because of duplicate names its giving me errors.

I could book each member of the band and each extra item seat separately but then there's a 24 euro administration charge each way

Does anyone know a way of getting round this so I can book all the flights at once like could I put "extra item seat two" or something?

Thanks in advance for your help