I also hesitate to say it's OK to take both, but the urologists I work with sometimes all seem to think it's OK.
PDE5 inhibitor side effects are in two categories. The most common ones are those which are drug specific and switching drugs can solve, and the less common ones are inherent in all PDE5 inhibitors. Heartburn (acid reflux) is unfortunately in the second category, because your esophagus and lower esophageal sphincter (which closes off the top of the stomach) are both smooth muscle contracted using PDE5, so PDE5 inhibitors can stop them working as well. This is not usually a problem, but is much more likely in men who get acid reflux anyway. Acid reflux over a long term can become a serious problem, so you might want to get this investigated if you do get it regularly anyway.
The muscle pain some people get is a Tadalafil-specific side effect, and this is because Tadalafil is also a PDE11 inhibitor. Sildenafil/Viagra and Vardenafil/Levitra don't inhibit PDE11. (Rather little is published about Avanafil/Spedra, so I don't know in that case.)