Translation:When we were opening our first shop, we were poor.
I believe the verb used in this sentence -- otevírat -- is imprefective, denoting an "in progress" or repeated action, so it is translated as "were opening." The perfective verb, indicating a completed action is "otevřít," which indeed would be translated as "opened."
It sounds ok to me. The speaker is emphasizing that the opening took some time.
The English is possible, but exceptional. An event expressed in the simple past (rather than a long-term condition) would be more typical (e.g., When we were opening our first shop, we had our second child.) "We were poor when we..." would almost always be followed by a simple past verb.