Hi OP.
I have two under two and my son is only just starting to speak. We're at the beginning of our journey so I don't have much to add right now. We live in France and I speak English at home but everything else is in French. There are English classes for little ones and bilingual schooling options but nothing much until age 3.
I mostly read to him in English, more than my husband reads to him in French, but I have just ordered some bilingual books from Amazon, the idea being that he can have the same story with the same illustrations in both languages. He mostly speaks single words at the moment and I think he has started to grasp that there are two words for everything. If my husband points to a picture of a duck and asks him what it is, he'll say "canard", and if I point to the same picture and ask him what it is, he'll say "duck". So I think he is starting to get it.
I'm hoping that the bilingual books will help with this, and if they are a success we'll try to get hold of as many as we can. There don't seem to be that many on the market though. It would be really amazing if publishers produced classic children's books in bilingual format, like the Tiger Who Came to Tea, the Very Hungry Caterpillar, the Gruffalo etc. Is this a thing? If not, it should be!
Flashcards sound a bit too much like formal learning, but he does have a couple of books of the "first 100 words" type format which have the words in both English and French. He seems to like those.
We are keen to avoid exposing him to screens for as long as possible, which obviously removes one source of potential language learning which we will need to replace with more live interaction.