Under Baseband it says
Gemalto PLS8 3G/4G modem w/ single sim on replaceable M.2 card
So I guess it should be possible to swap it for a dual sim one? I’m sure many would like a dual sim phone as well, and when they’ll find a way to hack the Librem 5 and do that, they’ll most probably share their experience with the community. 
I’m in the same boat. Dual SIM is essential for work purposes these days; one line for family and one for business on a single device has simplified my daily life a lot compared to lugging around two different devices altogether.
for what it wants and needs to be i think purism has made the honest choice regarding producing a libre smartphone device with only single sim on removable m2 card. considering their constraints on hardware and software design they have made a pretty good job with some exceptions regarding the battery.
a real dual sim ( simulataneos dual-network ) is a rare find and is not as energy efficient as a single sim one. the main reason beeing in that it has to simultaneosly send/receive on both networks thus making everithing a convoluted mess.
if you need to separate business from pleasure the obious choice would be two separate devices that allow you to remain active throughout the day without worring about battery life. if space and differentiation is a problem use smaller phones with different colors or different bumpers.
blackberry is a beast in that department because they have those sweet phone holsters that you can keep on your belt and are general work horses even if they are a little old (the physical keyboard type-ones with the hot-swapable batteries).
My Linux based BQ E4.5 (UBports) has got two SIM slots and I have two SIM inserted of different network providers here in Germany (O2 and Telekom). The latter one is a prepaid one which I do have for tests purpose only. I could disable the slot or withdraw the SIM if energy would be a concern. I don’t. Having two slots as well in the L5 would be nice, but is not a show stopper for me.
@purism:
Living as a Danish expat in Lithuania, a single phone with dual SIM is a must have for making and accepting those international calls free of charge. Most people prefer to dial a national number to an international due to phone company policies, even if no extra cost is implied (thx EU!).
The bottom line is that dual SIM is a must-have for me. If librem5 had dual SIM, I would order one today as my phone is aging and I’m looking for a replacement.
So makes it +1 times x for Dual Sim
So maybe Purism can still get it going. We could nag them on the chat as well 
You could nag but it won’t really help, and someone might suggest that you buy two Librem 5 phones instead. 
buy as many librem 5s you can carry on you MOLLE commando vest. equip a backpack with a librem 15 and your ready to roll…
It still is a real pity that they decided to go for single sim. Now the use of the Librem 5 for me decreased quite a bit.
I need two numbers. A work and a private one, which I certainly do NOT want to mix.
I am quite sad 
Now quite a while has past. Some fog has cleared and maybe there is already a roadmap for multiple SIMs?
Any news? As soon as Purism can produce a Librem 5 with 2 SIMs (e or otherwise) I am ordering one!
It’s still only 1 SIM, unfortunately. There are no plans for a dual SIM Librem 5 at the moment, as far as I know.
Could someone elaborate and put this in to perspective: what would it actually need on the HW side to have a dual SIM of any combination (physical and/or e)? Taking into consideration what has been confirmed since '17, how massive changes would it require? Can the card reader be used to read a SIM or does it need to be replaced? Is there a chip that’s needed to handle dual? Would it need dual radios or special radios? Does eSIM need a chip or just software? What are the different options, how big of a leap would this be?
I think the modem itself would require dual SIM support, never mind about the space to install a second SIM.
Another thought: On my android telefon I can use a separate mobile number using this app:
https://play.google.com/store/apps/details?id=de.sipgate.app.satellite&hl=de
So I hope something similar will be able with the librem5 sometime…
The e-SIM mentioned is interesting. That means no hardware changes are needed.