Extra Watts Dual mining is 27%.

R9 290 uses 45watts.

This costs (in UK) £0.10 more each day.
The extra SC (SIA) is worth £0.27 more each

Therefore, not really much reason not to Dual mine, plus there is an extra 2.5 Eth at pool each day.

Is that for one 290 card that you get £0.27 more each card? So the extract earning is £0.17 per card.

That might not worth the dual mining.