The last 36 months relief was reduced to cover the last 18 months only with effect from 6 April 2014 as you can see here.
If you live in the property for the last 18 months of your ownership of it, you haven't lived in it at any other time during your period of ownership and you each make a make a gain of £65,000 with the first £11,000 being exempt, you will each have a taxable gain calculated as follows:
Total period of ownership: 142 months
Exempt gain (due to occupation): 18/142 x £54,000 = £6,845
Letting period gain: 124/142 x £54,000 = £47,155
Letting Relief: £6845 (lesser of £40,000, £6,845 and £47,155)
Taxable Gain £40,310 (£54,000 - £6,845 (Main residence Relief - £6,845 Letting Relief)