How can I create mosaico block with 2 button?
This is not an answer for how to create a new block, but if you need something quick and dirty with two buttons side-by-side, hold your nose for this horrible hack...


Drag over a "Two-Columns Block".
Change to Block Level Style so you don't mess up other blocks.
Change the Paragraph Size to the smallest (or use the instructions in the next section).
Disable "Show Title" and "Show Image".
Remove the text from both columns.


If you want to minimize the blank text areas:


Select one of the text areas.
Choose the Source Code option ("&lt;>").
Substitute a placeholder: &lt;p style="color:transparent;font-size:1px;"&gt;&amp;thinsp;&lt;/p&gt;
Repeat for the other text area.


Unfortunately, there are no options for aligning the buttons or making them different colors, but at least you'll have a two button block to butt up against other blocks.

Notes:


I'm assuming you are using the Versafix-1 template.
I found that I had to use something other than an actual space character (or &amp;nbsp;) for the text areas. Otherwise, the source code block wasn't saved the next time I tried to edit it. I could leave some transparent text, but I don't want to impact someone using a screen reader.
You might be able to use "font-size:0" to completely eliminate the text area but I doubt you'll be able to select it again since there is no substance left for the editor to represent.