I not sure you can do no. 2 if you are Self Employed/Trading As even if it is a voluntary registration ?
I was told HMRC will treat income from any source as 'one' (and therefore would be assessed via a single self assessment form), thus potentially pushing you T/O above the vat threashold and thus registration required.
I understand it is possible if both business are Ltd companies are they are seperate entities, with seperate accounts (and costs...).
I am also sure there are some HMRC tax avoidance rules to stop you having, say, 10 companies all roughly in the same business sector and trying to keep each companies t/o below VAT threshold.
As they say; there are only two things certain in life - paying tax and death !!!