Can I burn .m4a and .mp3 file on the same CD?

Just to summarise the previous information. Data CDs are for storing/backing up your tracks (they don't "Play" as such) and can contain different formatted files. Audio CDs are automatically formatted to play in regular CD players (and will of course play programs such as iTunes and Windows Media Player for example). MP3 CDs are also "playable" they will play in some domestic CD players and in many in-car CD players. MP3 CDs will hold and play many more tracks than and Audio CD.

You can burn the different files to Audio CD which will automatically convert the files to AIFF format for use in regular CD players but to make an MP3 CD all the files have to be in MP3 format. You can't burn an MP3 CD from mixed formats.

Can both be burned onto the same CD via iTunes

Yes.

And if so, what format should both podcasts be in?

If you want an audio CD, just put all files into a playlist and click *Burn CD* then select Audio CD. iTunes will convert them to CD audio.

Selecting Data CD/DVD in iTunes will burn the files as they are. They can be MP3, AAC, AIFF or whatever and will be burned in that format (useful for backing up files)

If you select MP3 CD (this is the format of the files on the CD), the files must already be in MP3 format.
This is the same as a Data CD/DVD except iTunes will use only MP3 files (because many players can play MP3 files).