BA would be able to price this up as one trip but this would be classed as a multi city and not two returns, multi city may not price the same as the 2 returns. If its BA outbound I assume it's via LGW which will includes another connection.
Someone may know otherwise but a call to BA would confirm. If it does increase the price significantly, Ryanair may be a better option, especially if you are planning to overnight anyway.