[nerto]

Is it possible to install Kodi 17 and Kodi 18 at the same time?

[outcave]

For what I know: NO.

[zehner]

If you have an android box and your SOC is supported (e.g. S905, ...), you can install LibreELEC on a SD card.  So you can parallel boot Android (e.g. with KODI 18) from internal NAND and LibreELEC (e.g. with KODI 17) from external SD card. 

But I think thats not possible with a tablet or phone. I've heard of LibreELEC images for a few devices, but they all install to NAND
=> no parallel boot and therefore no two different versions of KODI at the same time

[Tinwarble]

No, they have the same signature and therefore can not be installed side by side.