wierd arp issue with 2 interafces sharing same subnet

This is due to the default configuration for the arp_filter option in the network stack. The possible values are documented in https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt:

arp_filter - BOOLEAN

1 - Allows you to have multiple network interfaces on the same subnet, and have the ARPs for each interface be answered based on whether or not the kernel would route a packet from the ARP'd IP out that interface (therefore you must use source based routing for this to work). In other words it allows control of which cards (usually 1) will respond to an arp request.

0 - (default) The kernel can respond to arp requests with addresses from other interfaces. This may seem wrong but it usually makes sense, because it increases the chance of successful communication. IP addresses are owned by the complete host on Linux, not by particular interfaces. Only for more complex setups like load- balancing, does this behaviour cause problems.

arp_filter for the interface will be enabled if at least one of conf/{all,interface}/arp_filter is set to TRUE, it will be disabled otherwise

Also arp_ignore is relevant here:

arp_ignore - INTEGER Define different modes for sending replies in response to received ARP requests that resolve local target IP addresses:

0 - (default): reply for any local target IP address, configured on any interface

1 - reply only if the target IP address is local address configured on the incoming interface

2 - reply only if the target IP address is local address configured on the incoming interface and both with the sender's IP address are part from same subnet on this interface

So, after setting arp_filter to 1 and arp_ignore to 2, you should get the behavior you want.