No, this could cause strange behavior, if we really get a new libx11. But =2:1.1.99.6+0m5 | 2:1.1.99.6+0m5-thumb1 should be possible. But IMHO the cleaner option is to change libx11-thumb to require the standard libx11-data instead of its own identical copy. See e.g. https://packages.debian.org/wheezy/libx11-data . If you want the thumb-CSSU to be complete on its own simply include a libx11-data without the -thumb suffix.