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Two 27'' monitors will take up a ton of space and you'll have to really turn your head to read from one screen to the next.
That's why I'd mount them vertically.
I've got 2 23" monitors (mounted horizontally) at the moment, so that's 40" wide (although they're in a slight V shape so probably 39" wide with 1" wasted on two 0.5" bezels) and 11.5" high and a total of 3840x1020.
Two vertical 27" 4K monitors would give me 4320x3840 at 23" high and 27" wide (so 26" in a slight V shape and adding a bit for bezels).
So less horizontal distance than currently, but twice the height. I was going to stick infrequently used windows (that I'd still like to reference) in the top half of the screens.
Greenbank-
If that's what you think will work best then go for it. If you do go the Iiyama route I used one of them for a long time without issue, I seem to remember they use unbranded Samsung panels. 4k is real nice, everything renders that much smoother, especially text.
Most monitors I've come across will only do 4k at 30hz if you use HDMI so stick with thunderbolt 3 or displayport for them.
Sumo
Looks like some people have had success using a Thunderbolt 3 hub for dual 4k monitors with your laptop.
https://forums.lenovo.com/t5/ThinkPad-T400-T500-and-newer-T-series-Laptops/T480-DCH-Thunderbolt-driver-amp-TB3-Dock-Firmware-upgrade/m-p/4352681?page=1
I'd have a think about whether one or two would be most useful to you. For example if you do presentations or virtual meetings with screen shares a lot then having separate monitors makes it easier just to share one and keep the other private.
Two 27'' monitors will take up a ton of space and you'll have to really turn your head to read from one screen to the next. I'd be tempted to go for an ultra wide monitor instead, something like the 35'' wide BenQ EX3501R. There's also no chance to lose your cursor between windows or bezel taking up space right in the middle. Not full 4k but good enough.