If you're afraid of losing your progress in Czech, then it's a sign that you may not be as confident with your level as you initially thought. There's a rather easy solution, though; find a few people to practice your Czech knowledge with on a regular basis (fun discussions about anything interesting, etc) while reading books/articles/newspapers in that language to keep your reading skills up, too.

An ambitious technique that I've heard polyglots use is that you attempt to learn your new target language by communicating in your previous one. So, essentially, you'd be taught Russian in Czech, which would build up your skills in both languages and decrease any fear of falling behind with either. Since you're still at A1 level in Russian, this could potentially work to your advantage; you don't have as much to re-learn while being tutored in Czech for them. And, who knows? You might reach C1/C2 in Czech by the time you get to B1/B2 in Russian...

Thanks for your input, René, but I would like to challenge you on some of the  points that you make:

Firstly, I appreciate that the two languages are not as closely related as most people think, but for native English speakers the fact that there is a case system and that they use perfective/imperfective verbs in a similar way is enough to confuse us and they are the reason I feel hesitant and wrote the post

Secondly, I can't agree with you when you say Czech is a useless language. There is a core of learners here on italki who learn Czech for a variety of reasons, be it for family heritage, working in the country, studying at a Czech university...Me. I'm just a language nerd who spent a happy year working in the CR 20 years ago and now that I'm retired I want to complete unfinished business and learn Czech to a high level. I like the sound of the language, I like the people and I get satisfaction from being able to communicate with people in their own language. I don't 'need' to speak any language other than English, but I choose to because languages fascinate me.

"Suddenly switching to different language when you can barely use the previous is not  agood idea."

A bit of a low blow that one, René...I'd say a solid B2 is pretty good and I'm sure most people on this forum would agree. Maybe that's not what you meant to imply - perhaps your level of English let you down?!

Thanks, Julia - it's interesting to know that there are others out there on the same journey! It'll be interesting to hear how much of a help or hindrance Russian becomes as you progress with Czech.The fact that you have a high level of German proves that you can master case systems, which has to be an advantage. My wife is a native German speaker so I maintain that language quite easily, so yes, experiment and see how it goes with learning Czech/Russian.

I just read your post in Russian and could actually understand a fair bit, which is encouraging. Klobouk dolů!

Hey, I am also learning both languages simultaneously! Though I am not as high in levels as you are. ><"

I am not an expert, but I can tell you what I have noticed thus far. Starting Czech, I thought, "Ha ha! I know Russian, so the beginning should be easy!". Wrong. While there are similar words (Ty, Vy, ect), It's not as many as I thought. I even found some songs in Czech and asked my native Russian pen pal how much he understood. He told me it was a nice song but he understood nothing. lol Yes, because they are both Slavic languages it is a bit easier to learn, but I don't think they are as similar as French/Spanish/Italian are with each other. 

I was also told that the Czech grammar will have some differences, so I have to "put my Russian grammar away." XD 

Again, I am no expert, and I am only beginning Czech. But I honestly think that as long as you continue Czech via conversation, reading, music, ect every day I don't think you will lose it. And if you do feel that Russian is taking too much time away from Czech, you can always take a break from it. :)

René: - Apologies for the confusion over levels; I personally don't feel the need to proclaim my language skills to the world  on my profile, so I just leave it blank or at beginner level. Thanks to those poets, I guess!

Yvona: Thanks for sharing your thoughts and yes, I think maintenance is the way forward. As an English native speaker with good French and Spanish, I don't worry about leaving Italian alone for 6 months while working on the other two. Slavic languages are another thing altogether ( at least for me) and having invested two years in learning Czech... 

Still I've now grasped the bull by the horns, dusted off some old Russian books, had a couple of conversations on italki and started to read and listen to Russian. I've not used Czech for three weeks but will have a conversation class soon and see what happens!