Could anyone advise how to go about connecting one or two xf300 to a projector i.e. a live stream?

Ian, I think HDMI would be the way to do it. Although I cannot recall if the HDMI output on the XF300 is clean... or cluttered.

What inputs are available on the projector?
What input resolutions does it support (older projectors may not support HDMI and/or HD sources)?
Do you need to be able to seamlessly switch between the 2 cameras?

A projector hasn't been purchased yet, so advise on that would also be welcomed. Essentially, the ability to switch between a close up view and wide view easily would be very useful.

So...Is it as easy as just using long hdmi cables, so long g as the projector accepts them? How about the switching, how is it done? What about audio?

no problem if the projector has an HDMI in. The XL 300 Can output clean video with no problem. your audio feed will be fine, and in the past I have taken audio in from the soundboard directly into the XLR jacks and it works fine. questions ?

Live streaming out to the Internet might require additional gear depending on how you are doing it. there are a couple of different ways, but for example black magic design now has a box if you just plug in either SDI or HDMI and you can stream to the Internet directly, like using a WebCam. but it sounds like you're talking about a live stream at an event not over the Internet

Details will depend on your intended use/venue; e.g., small church, large auditorium,classroom, and ability to control light where the projector is used. For two cameras and seamless switching on demand you may want something like:

https://www.bhphotovideo.com/bnh/controller/home?O=&sku=1198625&gclid=CLyRlvWBmtMCFQiHswodAtwH4w&Q=&ap=y&m=Y&c3api=1876%2C%7Bcreative%7D%2C%7Bkeyword%7D&is=REG&A=details
or
https://www.bhphotovideo.com/c/product/1304648-REG/datavideo_se_650_hd_4_channel_digital.html

There are other HDMI-capable switchers/mixers on the market as well. (Note I have not used any of them, so read user reviews before you buy.) And if switching two cameras you may still want to keep a single audio feed. Most mixers should be able to accommodate that.

And be sure the projector supports the output resolution of the XF300 that you intend to use and has HDMI inputs. You also should be sure that the projector's throw distance and brightness are appropriate for the venue where it will be installed.