Q: Can I submit two similar papers to the same journal?
I submitted a paper to a Springer journal on computer science on 5/10. On 10/10 the status changed to "Editor assigned" and has remained unchanged ever since. I emailed the editor in Dec and he said that they are looking for reviewers and I have to expect 4-5 months of reviewing (journal speed review is 88days!). On 20/12, I received an invitation to submit an extension of a second work to a special edition in the same journal (only + 30%new content), deadline 1/03. The two works are very similar but with different presentations and results. I'm working on the extension. Should I withdraw the 1st paper?
Your question is not very clear to me. You have mentioned that you've been invited to submit the extension of a second work. However, you have then stated that this work (the extension) will be similar to the first work that you have submitted to the journal. Does this mean that three of your works will have similar content? That would appear slightly odd and might raise concerns of salami slicing.
If you feel that the extension and the submitted article might have a lot of similarities, I would suggest publishing any one of them. If you think that publishing in the special issue would be more prestigious, it might be a good idea to consider withdrawing the other manuscript from the journal first.
Also, please reconsider whether you would like to publish the first article at all and if publishing the extension makes the first article redundant or repetitive. Please think through very carefully before submitting it to another journal.