How do I model a process which can start by either Role1 OR Role2 performing the first task? I would not like to use "Role1 or Role2" in the lane description
The BPMN specification does explicitly not specify the exact purpose of lanes (p. 304/335 in PDF).
Consequently, the approach Role1 or Role2 is actually compliant with the specification. The approach is also not far away from good practice. Adding multiple roles and even individual users as candidates to a task is often possible when configuring processes in BPX (Business Process eXecution) systems.
Typically, you would create a new role that encapsulates both Role1 and Role2 - let's call it Role1;2. This makes sense from an information systems perspective. For example, if you execute the process with IT support, you need to grant Role1 and Role2 access to the system/component that allows executing your first task. When configuring the access rights, the administrator will create a new user group - Role1;2 that contains the the user groups Role1 and Role2.
Note that a process role doesn't necessarily map to a job title. For example, a person who is employed as a Software engineer might share the process role Specification writer with a System analyst.