I don't have a simple answer, but some things to consider:
With (very roughly) a third of the bandwidth available, things will take 3 times as long to download. Upstream bandwidth is likely to be very much lower, so uploading will be considerably problematic. Is that acceptable for the duration?
Without knowing the details of your setup (is the 40Mb Fibre connection FTTx/Cable? Or a symetric Leased Line with high availability with robust SLA/SLGs?) the ADSL service may very well follow the same path from the exchange, so both services would be susceptible to a navvy with a JCB if he were to damage the cable/duct as there will be no seperacy between the two services.
If (say) your existing service is FTTC (and so within the same cable as the ADSL service) and this cable was damaged and took several days to repair, what impact would it have on the business? How important is internet access for your work? What will it cost in lost productivity if it were to be lost for a while?
Once you understand the risks and potential financial impact, you can better decide if it's worth paying for a more resilient solution.
Or you may decide the risk of losing both simultaneously is low and the loss if internet access isn't critical for the business, so go with a low cost option.
Eats shoots and leaves.