It would be clearer thus: "Brian proceeds to invest in Cliff's portfolio in a manner similar to how he invested in Martin's portfolio. This is based on the fact that both brothers have similar lifestyles, and that they are only two years apart in age."
In both the examples you provided the portion which is confusing is missing a reference to the past. If you prefer your wording, you need to adjust it just a little to "in a manner similar to that in which he invested in"
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