Can I pay for my one lot of shopping in Tesco's with 2 separate debit cards.?

If you go into a shop, yes (especially if you split the shopping into two at the checkout). Online, no.

yes just put them in two separate piles and say ill pay for them separately

depends on the cashier and the policy of the store

Thanks Grumpier Old Woman. It's a debit card so it my money - not borrowed like a credit card.

Yes you can, if you must, and I do empathise, but you don't need me to tell you that it's a very expensive way to do your shopping, if you can't pay off the whole balance when it's due. Probably, if it's your only option, it would be easier to put two separate lots of shopping through the check-out, paying for each one with different cards. It's less embarrassing and easier for the cashier to organize.