there are two ways of doing it. (I was around in the sixties lol still think its amusing when its called "old fashioned like the victorian era) Anyway you make up enough victoria sponge mix to fill two sponge tins. If you want a chocolate and vanilla harlequin then make half the mixture according to any basic chocolate sponge recipe, this usually means having a certain amount of cocoa in the mix and less flour. If you just want it two colours then make one big batch of batter and divide it in half. lets call the colours pink and yellow.
Colour the batter, mixing in the colouring well but gently and add flavouring if you wish.
Method one. Using a dessert spoon but neat blobs of alternate coloured batter around the edge of each greased and lined tin. When you do the next row in, put slightly smaller blobs of batter and ensure that a pink blob goes against a yellow blob and vice versa so you get a nice chequered pattern. Do both cake tins in the same way and bake according to your recipe. When the cakes are cooked and cool, make up two lots of buttercream or frosting in the same colours. Blob the frosting onto the bottom layer of the cake, blobbing pink frosting onto the yellow bits and vice versa. Put the top layer on, rotating it slightly so yellow cake goes over pink frosting blob.
Decorate the top, et voila.