Hi Francis,
Your wife only needs to spend 2 years to qualify for ILR as your dependent, so it does not matter if she arrived in UK 6 months after you did. The two of you can and must apply for ILR together using the form SET(O). This will only save you money. If you both apply separately then Home Office will charge you twice the fees. Plus, you both will have to pass the Life in the UK Test.
Hope this helps.
All the best.
Cheers
Navneet