Well, That actually helps out to answer your question A LOT, @MouSkxy will be able to give you some good pointers. @DeviantOllam is on this forum from time to time.
He has an xEM in one hand and an xNT in the other, So he can recce access panels, by walking past and swiping and if you get a response, you get an idea of what Frequency you are working with ( I will find you the link from where he mentions this ) (You could use the NExT, but it could cause you issues on a dual-frequency reader) SOME of my work readers do this to me on occasion, which is why I am putting in a specific xEM dedicated to my workplace.
Here are my initial Thoughts
BUT The problem you have is, you will need to (should) plan ahead your placements
NOW, There are sooo many options and variations you could choose, so I will give you one idea, and let @MouSkxy and @DeviantOllam correct me.
Get 2 x xLEDs an LF xLED in one colour and HF xLED as another ( I have Not so subtly, hinted at a 2in1 xLED device for this purpose )
The bonus of this is, using an xLED you will visually, silently and with no interface with the actual system ( It is just energising the LED ) so you can at least get a Frequency, and specifically sniff for the employee cards on a “War walk” with the correct antenna.
Placement, 2in1 ideal, but until then ( if it happens ) you could go HF & LF in the same hand ( at least 5mm apart for safety) or one in each hand.
Argh, I said one suggestion and to many variations, oh well…
So NOW you have your Frequency IDentifier solution, you need something to write to ( UID/ NUID)
Fortunately, your options are made easy by the fact that you only have a couple of options to choose from.
Low Frequency
xEM ( Compatible with may systems )
NExT ( May cause cross talk, although I love 
the NExT, in a Pentest, I dont think it is the best tool.
High Frequency
xM1 ( soon FlexM1gen1a & FlexM1gen2 - maybe x Series also??? )
as I said in the other post, This is only compatiable with a specific system, BUT if your target uses it, you are onto a winner.
Now we know your Pentesting career pursuit, I will amend my initial recommendation
Your shopping list is as follows ( for what is currently available in the DT store )
xLED LF Colour a
xLED HF Colour b ( You could get xSIID, PRO storage and access CON potentially not invisible to security system you are trying to stealthfully interrogate )
xEM
xM1
+
Spark 2???
You could get more, but that is a good start
Apex When it is available… but now I am getting distracted.
But since I am distracted 
In the future I would wish for 
a Pentest Bundle
2in1 LF/HF xLED + xEM + xM1
OR
I am hoping for a 2in1 xEM/xM1
OR
2in1 xEMLED 2in1 xM1LED ( But would you put HF LED with xM1 or LF LED etc???
Distraction finished
Now where to place your current shopping list?
again many variations, But this time 
one suggestion ( you could argue for the other variations, but at the end of the day it is YOUR decision )
Left Hand LF & HF xLEDs
Right Hand xEM & xM1 
I’ll leave it at that,
Hope that is helpful and some food for thought.
any more questions, feel free to ask, Myself or the others Should be able to answer them.