We have already showed you how to Multi boot LG Nexus 4, and LG Nexus 5 seems to be coming up with the same power as Nexus 4. Developers have put a lot of Custom Power in Nexus 5 and with the Landing of Multi ROM for this device, things are taking an interesting turn. For those who are not Familiar with Multi ROM, it is an application that allows you to Boot Multiple ROMs on your Android device. So, Far Nexus 7, Nexus 4 and Nexus 5 have supported this. For now you can boot your Nexus 5 with most Awesome ROMs available for it.
Furthermore, you are not restricted by only Android Based ROMs, you can even install Ubunt Touch or FireFox OS as your secondary OS on Nexus 5, as soon as they are available for Nexus 5 you can install them, till then you can do good with Android based ROMs. Installing the ROMs on it is as same as installing on Nexus 4. Follow the Link to find out how.
Again if you want to install Ubuntu Touch and FireFox Os on your Nexus 5, you will need to wait for their official release. Till then you can wait or follow the Guide to install Multi ROM on your Nexus 4 or using the same tutorial, you can install Android Based ROMs on your Nexus 5 as Secondary OS.