Insert rows into a table from another two tables separately in mysql

Fiddle Example

I'm planning on a price alert system to notify users once a product hit lower than their target prices. Some circumstances confine me to store the mainstream and second hand retailers' prices in two separate tables. I may not be able to change it in the near future so forgive me if it may complicate the issue. To notify users once a merchant's price, (whether it's mainstream or second hand) reaches a user's target price zone, I need to insert a record into table price_alert with values entry_id,user_id ,merchant_id and lowest_price. Here's my question. Can I insert records into price_alert from table mainstream and second hand separately in one statement?

My expected output should be like this:

ENTRY_ID    USER_ID  MERCHANT_ID    LOWEST_PRICE   Is_read
1           1        3              100            0
3           2        1              300            0          // Merchant 3 is a second hand store while Merchant 1 is a mainstream store.

This code won't work because it can only get the price from the mainstream_retailer_price

INSERT INTO price_alert (entry_id,user_id,merchant_id,lowest_price)
SELECT
u.entry_id,mrp.merchant_id,u.user_id,mrp.price
FROM 
 user_target_price u 
 INNER JOIN mainstream_retailer_price mrp
  ON u.product_id = mrp.product_id
 INNER JOIN second_hand_retailer_price shrp
  ON u.product_id = shrp.product_id
WHERE
 (u.target_low_price > mrp.price)
 OR u.target_low_price > shrp.price
GROUP BY u.entry_id

Can I do something to the effect of:

SELECT
u.entry_id,(mrp.merchant_id OR shrip.merchant_id),u.user_id,(mrp.price OR shrp.price)

Table Schema:

CREATE TABLE mainstream_retailer_price
    (`id` int, `merchant_id` int,`product_id`int,`price` int)
;

INSERT INTO mainstream_retailer_price
    (`id`,`merchant_id`,`product_id`,`price`)
VALUES
    (1,1,1,200),
    (2,1,2,300),
    (3,2,1,150)
;

CREATE TABLE second_hand_retailer_price
    (`id` int, `merchant_id` int,`product_id` int, `price` int)
;

INSERT INTO second_hand_retailer_price
    (`id`,`merchant_id`,`product_id`,`price`)
VALUES
    (1,3,1,100),
    (2,3,2,600)

;

CREATE TABLE user_target_price
    (`entry_id` int,`user_id` int, `target_low_price` int,`product_id` int)
;

INSERT INTO user_target_price
    (`entry_id`,`user_id`,`target_low_price`,`product_id`)
VALUES
    (1,1,150,1),
    (2,1,200,2),
    (3,2,350,2)


;

CREATE TABLE merchant
    (`merchant_id` int, `merchant` varchar(20))
;

INSERT INTO merchant
    (`merchant_id`,`merchant`)
VALUES
    (1,'First Hand A'),
    (2,'First Hand B'),
    (3,'Second Hand A')


;


CREATE TABLE price_alert
    (`entry_id` int, `user_id` int,`merchant_id` int,`lowest_price` int,`is_read` int)
;