How would you treat Stamp Duty for % house ownership

[GasPanic1]

I'm not sure if legal is the right place for this or property but here goes.

I'm buying a property with my partner. I already own my property. If you take the purchase price and our contributions both now and going forward I would own roughly 20%. However, as we have higher rate stamp duty to pay that significantly reduces my share as the overall cost goes up.

When deciding on shares for ownership should the SDLT been a consideration on top of the purchase price. I have, naturally, paid SDLT on the current property I own and will be keeping and gaining a very small income from.

Ultimately, we will both benefit long term from this property and the one we are to buy too.

Base the split on the total cost of purchase price + stamp duty

Who is actually paying what though?

Is the stamp duty increased because you already owning a home I presume it will be classed as a second property
you should pay the extra that’s incurred because of your circumstances