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Most star map software like Starry Nights, Sky Safari etc let you enter telescope and lens parameters to show a box in the program showing you the coverage of the equipment combination. so you should be able to just use the focal length of the lens in a telescope parameter (they are the same basic things really). So I suggest you use the software you have and input the lenses you have and go view the region with the two objects (guessing comets) to see firstly if they fit in the field of view or not.
Ideally I suggest you use the rule of thirds to try to find a lens where each desired object is one third in from opposite corners. Not just for symetry but because you are unlikely to get the midpoint between them dead center of the frame, so if you try to have each only just inside the fov you probably wont get both.
Also ALL lenses distort towards the corners dramatically so you really dont want target objects in the corners for that reason alone. the closer they are to the centre of the frame the less distorted the stars and everything else is.
With that in mind you could look at taking a mosaic of frames with the targets in centre frame and other frames to cover the whol region to stitch them all together then you can crop to a single shot with the two targets in opposite corners. This gives you the least optical distortion of the objects.
I understand if you want to capture both in one shot for whatever reason. I did two comets in one shot, not a spectacular photo but challenging one Im proud of. So do it virtually first in software and have a lens that covers the targets comfortably, not at a pinch. if the objects are too faint to see in live view try to find a bright star close to one or both or dead center between them to help you orient your view. You'll need a ball head mount on your tripod so you can orient to whatever orientation is needed. Using the star software you can also play with position the objects in say topright and bottomleft of field of view and then topleft and bottomright. you might find one composition is more interesting than the other or complementary to highlighting the targets. You should also do test shots to make sure you can actually capture that patch of sky when the time comes. it also helps you to know what little groups of strs should be in all the corners and have that on paper so you can line it all up and take some test shots to check. maybe you want to get two iridium flares in one shot so test shows are always good. it helps make sure you have exposure settings correct to actually capture the targets, and they aren't streaking etc. ISO really should be as low as possible unless its nebulosity you want to shoot so you need to test yourself what YOUR actual camera noise is like at the time/temp and whats acceptable to your shot, around 400/800 is a good test starting point. Is your shot intended to be taken in camera or are you going to post process or stack frames? stacking lets you bump up the iso a bit since it gets suppressed but one shot from camera lower iso is better always provided you get your targets exposed. if the targets are bright you can also reduce your fstop down to f4 to sharpen up the image and reduce some of the optical distortions too.
Since you cant give more detail you'll have to figure out from the advice what your answer is. it all depends.
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