I think I know the answer to this question but will ask it anyway as life is
often full of surprises!

My client data is held on an increasingly large spreadsheet - into which I
have built some really useful functionality using conditional formatting,
links to other spreadsheets etc. It works extremely well for what I need.

However, I would now like another person to be able to access the
spreadsheet with simultaneous read/write access. I know a second person can
have 'read only' access, but is multiple read/write access possible by any
means please?

If not, presumably the only option is to change over to using a proper
database - but I would prefer no to do this for several reasons if at all
possible?

I would really appreciate any advice or suggestions on my predicament please

TIA

V