Different themes for different devices.

It's possible! I think this will take a lot of work but it's certainly possible. Here's an outline of the steps involved:

1. Fork a copy of the magazine template then adjust the CSS of this copy until it visually resembles the Portfolio template. This process will be hard, since you won't be able to manipulate any of the HTML – just the CSS. I'd expect this to take me a few weeks, at least.
2. Fork another copy of the magazine template then write a media-query which loads either the original CSS, or the CSS you created in step 1 depending on whether your reader is on mobile or desktop.

I don't think the end result will be good for your readers – if it were up to me I'd just make some tweaks to one of the templates on either mobile or desktop – but if you have the will to put in the work to accomplish the above, please don't let me dissuade you