Though you do not give the full context, it is likely that this starts rather than continues a point, so "insofar as" would be a standard translation of "que tant". The normal way of following that up by emphasising a constituent part is "and more particularly".

"described above, insofar as they involve silicones and more particularly silicones fabricated into breast implants, are highly controversial. ..."
jba.sagepub.com/content/8/3/237.full.pdf

"Insofar as the IRS and, more particularly, the Tax inspectorate have access to information on the income, assets and deductible expenses of taxpayers, ..."
www.coe.int/t/dghl/monitoring/.../GrecoEval1(2000)3_Sloveni...



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Note added at 1 hr (2010-12-09 12:36:45 GMT)
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"imminent problem for the industry' future labour needs, especially insofar as prospective construction workers, and tradespersons more particularly, ..."
www.cfmeu-construction-nsw.com.au/pdf/06unisydrpt2.pdf

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Note added at 3 hrs (2010-12-09 14:20:14 GMT)
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I didn't suggest "even more so" because "les époux X" - or at least one of them - appear to be included within the first term "MM. X et Y", so "more particularly" seemed more appropriate.