Related to What is expected shared cM of double third cousins?, what would the expected range and average shared cM be? In this case, I'm seeing 64.0 cM over 3 segments. Looking at the table, that is less than half the expected average (433 cM) and nearly outside half the range (115 cM to 753 cM). In this case, the further (1R) path includes a terminal paternal link that is a undocumented father with family stories giving the link.
Does this cast doubt on the half 1C1R and perhaps suggest it is more likely 2C1R (and no longer halved), that is the father of the illegitimate child was not the other's grandfather but instead his brother? In such a path, the cousins most recent shared ancestors are now a married couple, hence no more need to halve the relationship.
Source: ISOGG Autosomal DNA Statistics