You will be better off making two separate mixes, and making two marble cakes - so half mix of each recipe in each cake.

Making 2 mixes together is never really a good idea, but you may want to try doing it in a broader pan as opposed to a higher one. brush the top of the cake with water before you put it in the oven, and if it looks like it is getting too dark on top before the centre is cooked, cover it with foil halfway through.