This is interesting. I am also working on Spanish and Russian at the same time.
But I have been studying Spanish for 5 years and Russian for about 1 year, so Spanish is more "maintenance and improvement" whereas Russian is a newer project.
I do have some thoughts about the challenge of 2 languages at once.
1) It helps if the writing system and grammar are very different (you have this base covered)
This helps a little in keeping them separated in your mind.
2) I split the day, meaning one language gets studied in the morning and the other at night.
3) I use different methods for each language. So if you are using a flash card system for one language, use something else for the other one, etc.
4) Have clear goals for each language that are specific and also have smaller milestones to move towards and celebrate.
I am happy to discuss this further.