Hi Carlion,
Bit tricky this, and a lot of work on your behalf is involved if you want a multi-car policy. Sometimes this works out far more expensive doing it this way - especially if the two cars are very different. You may be better off doing a few quotes for multi-car and separate insurances. Admiral do a multi-car policy but I wouldn't insure anything with them myself, personally.
As one vehicle is only needed for a short term period, this also makes me think that this shouldn't be put with a multi-car company. You should try doing a quote on here for it then calling the insurer and seeing if they do any shorter term policies. Sabre/Provident etc. do 3 and 6 month policies through brokers. This certainly won't be exactly half the price of a full policy as the insurer's costs will need to be covered, but it will not be as expensive as a full year.
What you are asking for is 'non-standard' and unfortunately the internet quote engines are really designed for basics (at the moment!!). By all means run through the quotes, but ensure you speak to somebody over the telephone to ensure you have the best price and cover required.