Seems like you found a bug. Get the same error with my HANA tools installation (v2.2.8).
Although both parameters are supplied, the generated SQL only considers only the second one.
Generated SQL:
SELECT TOP 200 min("DATE1") AS "DATE1", min("DATE2") AS "DATE2" FROM "_SYS_BIC"."test.misc.model/CVScripted_DateInputRange"('PLACEHOLDER' = ('$$IP_DATE2$$', '2016-04-30'))
I tested also the web-based tooling (HANA Web-based Development workbench). This works fine. Both entered parameters are considered and the view can be successfully executed.
Searched for an OSS note, but could not find anyone related to the topic. Maybe you should open a ticket if no one else can bring some light on this.
As workaround you can either use the web-based tooling or do the select with all parameters by yourself.