It's not uncommon in Tartus, but far less common in Homs and Dimashq. Oddly enough it's usually Sunni husband and 'x' wife.

For example we have Sunni/Shia, Sunni/Druz, Sunni/Christian and most recently Sunni/Alawite in our extended family, all from Tartus.

Our relatives from Homs/Dimashq are almost exclusively Sunni, religious, and tend to marry from their own sect and even city.

There are irreconcilable differences between (for example) Sunnis and Shiites that would usually prevent marriage if the husband is 'stricter' in faith, not to mention the rift caused during family gatherings by the politics behind each sect (ie: Alawites are generally pro-Assad, Shiites are usually pro Hezbollah, Sunnis are usually against both)

Edit: This is obviously a gross oversimplification, basically less religeous more intermixing.