So far my experiences have been:

• You use different languages in different situations, this means that yes, technically, your proficiency in your languages is lower than a monolingual speaker in either of your languages because you don't use the same language in all situations. This is especially evident when I have to use a foreign language in an unusual situation. Not having the required practice can have a negative impact. But that's true for every language speaker, I remember I used to help my mother out when she had to write a letter of request because I had a lot more experience with composition back then.
• Language interference is a very real phenomenon and it can happen in all directions. Your L1 will obviously influence your L2 a lot more at the beginning but when you get about equally proficient in both of your languages, the L2 will start having a very strong influence on your L1, especially if you use that language more in certain situations/in general. Sometimes when you're in the middle of explaining something different words from different languages come to mind; thus, code switching.
• Using expressions that are common in both languages is also a phenomenon that I've experienced. There are many expressions in both languages (especially when it comes to prepositions/suffixes) that are okay-ish in both but wouldn't normally be said by a monolingual person. After 15 years of speaking my first language, I was surprised to learn that people here aren't honest with someone, instead, they're honest towards someone. Both forms are common enough but a small experiment revealed that the first one came to everyone's mind first but no one really frowned upon my version either.

Obviously, this is only true if you actually use your languages frequently enough for these phenomena to happen.