plant too big should i cut it?

Basically I have 2 plants in 1 pot and 1 is a autoflowering plant and the other was a fem seed, the autoflowering plant is around 2 weeks away from harvest however the fem seed plant is around half the size of the total height of my grow tent and i am concerned with how big it is because i am still doing 18/6.

Should I take a cutting from the fem plant and then kill the main plant or should I just switch to 12/12 now?

Any advice is appreciated.

yes, take some cuts from the photo they will be female clones and you will have a great auto plant and some great clones to fill your tent with. you will get great plants from the clones too. you can harvest more bud from a tent full of clones then from a single plant.
so leave it on 18/6 and get yourself some rockwool cubes and a humidity dome of some sort

something like this is all you need. rockwool starter cubes, they come with there own plastic tray, look for the ones in this plastic tray, you can cut it up to fit your mini propagator. and a high dome propagater like the one in the picture. that will give you enough space for maybe 6 clones