The trouble is that an examiner has to sign off both the SEP & TMG rating before they expire to revalidate them.
If you're revalidating by experience the 'experience' part has to be "within the 12 months preceding the expiry date of the rating". If (at the extreme) your SEP & TMG ratings expire 12 months apart, every year would be a revalidation year, one year for SEP, the next year TMG. Revalidation by proficiency check has to be done within 3 months of the expiry of the rating.
How much of a nuisance that is depends on your circumstances.