What a two year rent advance really costs you
The headline rent is not the number that leaves your account

Two years up front is still normal in much of Lagos, and the Lagos State tenancy law's limits are honoured more in the breach than the observance. Whatever the legal position, the practical question is what the arrangement costs you.
Price the money, not just the rent
Money you hand over in month one for occupation in month twenty is money you could have held. At any realistic rate of return, two years in advance on a mid market flat costs meaningfully more over the term than a slightly higher rent paid annually.
The costs that ride along
Agency fee, commonly ten percent.
Legal or agreement fee, commonly another ten percent.
Caution or damage deposit, refundable in theory.
Service charge, often a full year in advance alongside the rent.
On a two year term those extras land almost entirely in the first payment, which is why the cheque is so much larger than twice the annual rent.
Compare tenancies on total cash out in year one, then on total cost over the term. The ranking often changes between the two.
What is negotiable
More than most tenants assume. Landlords with a vacant unit and a mortgage are frequently open to one year at a premium, or two years with the second year's service charge deferred. The worst outcome of asking is a no.