You've posted on a UK-based site, so it's unlikely that anyone here is familiar with Iowa state law.
However a quick Google search suggests that, while some people are campaigning for a change in the legislation, Iowa definitely requires two licence (UK spelling!) plates at present:
http://whotv.com/2018/02/09/proposed-bill-would-require-only-one-license-plate-on-iowa-vehicles/
I can't find anything to suggest that out-of-state vehicles are exempted from the legislation.
All 28 member states of the European Union and, to the best of my knowledge, almost every other country in the world requires front and rear registration plates, so those states in the USA which don't have such a requirement are definitely out of line with the general thinking on the topic. (Here in the UK there are extremely strict rules about the size and spacing of letters and numbers on registration plates, with only one clearly-defined font permitted and with the colour and reflective qualities of plates having to meet extremely exacting standards. We tend to regard the lax standards of US states as a bit of a joke).
Your Q&A websites also don't seem to be as good as ours. For example, both Yahoo Answers and Answerbag are poorly moderated and, as a consequence, full of spamers and trolls. The only US-based site I can suggest that you might find a definitive answer on is Quora:
https://www.quora.com/