I think that if you do that, more of the strength will be along the epoxy joint rather than pulling one piece away from the other. Might be a bit harder to brace it tight for the curing process, but this is a solvable problem.
Yes, flat on the wall is better than being pulled down. A wide board is better than a narrow one - I would suggest a 1x6. The curtain rod should be mounted as close as you can manage to the board, and low down on it.
Clean the cement thoroughly before starting - if you can manage it, pressure wash, otherwise scrub thoroughly. If there is any paint, try using a paint remover to get it off.
For the adhesive, take a look at the Liquid Nails web site and see what they recommend for a wood to masonry joint. It might also be an idea to cut the wood into shorter horizontal sections in case the wood shrinks in a different way from the masonry as it ages.